[lit] Remove undefined keyword from an MCJIT test

Changed 'XFAIL: mcjit-ia32' to 'XFAIL: i686 && windows' because the
'mcjit-ia32 feature isn't defined anywhere, or used anywhere else.

I found this as part of other work to clean up lit keywords, so
I figured I would go ahead and fix it.  Verified by hacking my
lit.site.cfg.py so host_triple = target_triple = "i686-pc-windows-msvc"
and the test correctly reported XFAIL, while "i686-pc-linux" passes.